1. Field of the Invention
The present invention relates to a diffusion code generating method for a code division multiplex communication by a spread spectrum system.
2. Related Background Art
In a spread spectrum communication, there is considered a code division multiplex communication system in which by using low cross-correlation characteristics of diffusion codes, a plurality of communication paths are multiplexed in the same band and a information transmission speed is raised.
Especially in the code division multiplex communication of a synchronization system, by using orthogonal series in which cross-correlation values are equal to zero as diffusion codes, an interference between the codes can be completely eliminated. For example, there is a code division multiplex modulation/demodulation apparatus of the type in which a synchronization clock is individually given by a wire system by using orthogonal codes comprising M series.
In a wireless spread spectrum communication, however, a demodulator has to obtain a code synchronization from the reception signal. Therefore, a receiver generally has a correlator, a diffused reception signal and a diffusion code for synchronization are input to the correlator, and an autocorrelation peak of an output waveform of the correlator is detected, thereby capturing and holding the code synchronization of the diffusion codes.
In the case where a communication path is multiplexed by a code division multiplex by using a plurality of orthogonal codes, the periodic cross-correlation between the orthogonal codes which are used for synchronization and the other orthogonal codes appears as noises in an output of the correlator. Generally, there are positions where the periodic cross-correlation values of the orthogonal codes are not equal to zero at positions out of the synchronization point (when the phase is deviated). When the number of code division multiplexes increases, the cross-correlation values from the other channels for the synchronization channel are accumulated and the noise level increases.
In the communication by the PSK modulation, the accumulated cross-correlation value is largely fluctuated by the information bits of each channel. As a result, the output of the correlator on the reception side has large values at random at positions out of the synchronization point, so that it is difficult to discriminate the autocorrelation peak of the synchronization channel.
Now assuming that the code synchronization could be captured, in order to hold the synchronization after that, a synchronization point (or sync point) must be clearly identified near it in the output of the correlator.
For example, as shown in FIG. 8, at a point which is away from the synchronization point by one chip, when a cross-correlation 202 which is larger than an autocorrelation peak 201 of the synchronization channel (or sync channel) appears from another channel, a width of the peak to discriminate the synchronization point is widened like an output of a correlator 203.
Further, since the cross-correlation 202 largely varies by the information bits of each channel, the width of the peak or a signal intensity changes, so that it is difficult to always stably discriminate the synchronization point from the output of the correlator.
As mentioned above, even after the synchronization was established, when a large cross-correlation from channels other than the synchronization channel appears in the output of the correlator at a position near the synchronization point, it is impossible to hold the code synchronization or a demodulation error occurs due to a time-dependent fluctuation in the output of the correlator. Since such phenomena become typical as the number of code division multiplexes increases, the multiplex number cannot be set to a large value in the conventional code division multiplex communication and, consequently, it obstructs the realization of a high transmission speed.